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A.2
The Sound Level in the Coupler – Using an External Signal Generator
The
and
Microphone
external signal generator is used. Therefore, all corrections have to be added manually to the
observed results using the formula given in the previous section.
In addition, frequency response corrections have to be made for the reference microphone
embodied in the coupler. This is because the compression loop works from the signal coming
directly from this reference microphone, which is not truly linear. If it were truly linear, like a
laboratory-standard microphone Type 4180, no frequency response corrections would be
necessary even when using an external signal generator.
Frequency response corrections are made automatically at each discrete frequency when
Type 4226 uses its own internal signal generator, but not when an external generator is used.
Therefore, they must be applied manually. Each Type 4226 and its coupler is a matched pair
adjusted so that the sound level generated in the coupler is correct. These adjustments correct
for any departures from the linearity of a Type 4180 microphone.
These frequency response corrections can be determined quite easily using the external signal
generator and a sound level meter acoustically coupled to Type 4226. Proceed as follows:
1) On Type 4226: set the following:
•
Function
•
Sound Field
•
Sound Level
2) Select a linear frequency weighting on the sound level meter.
3) For each discrete frequency selectable on Type 4226, compare the rms reading obtained on
the sound level meter using:
a)
the internal signal generator
b)
the external signal generator
The observed difference
correction to apply.
switches do not influence the sound levels in the coupler when an
Sound Field
switch to Calibration
switch to Pressure
switch to any convenient Lin position
a)
b)
–
for the given frequency will be the frequency response
APPENDIX A
Corrections and Adjustments
35
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